Important Notice: Issued by Ambient Pressure Diving Ltd.

To be sent to all Inspiration owners: (by email and post)

21.03.2002

In 2001 many divers experienced "cell warnings/errors".

Cell warning, or Cell Error as the alarm was called on the first two years or so production, occurs for a variety of reasons and is an excellent safety feature, warning the diver when one cell deviates from the average of the other two by 0.2 bar or more. Cell warnings may occur during fast PO2 changes, fast ambient pressure changes, by obscured sensor faces, by decaying or faulty oxygen cell(s), by faulty connection between Master and cell or between Slave and cell. It is the only warning you get, other than over-active solenoid operation, that you have two faulty cells stuck low simultaneously.

The fault can be in the cell, it can be in the hardware or wiring, or it can be in the stored calibration factors for any of the cells  but the important thing is that the diver does receive a warning.

In 2001 we experienced problems with the R22-2BUD oxygen cells.  By swapping the wiring to two of the cells, you can easily identify whether it is the cell that is causing the cell warning or whether the problem lies between the Master and cell or Slave and the cell.

So, it was easy enough for us to find out that it was the cell giving the problem. The difficulty lay in proving that it was the cell that was causing problems. The cell's outputs in air and oxygen were often completely normal and the cell's reaction time to changes in PO2 was quite normal, so proving it was the cell was quite difficult.

By subjecting the cell to rapid changes in ambient pressure and with the help of John Lamb at Vandagraph we were able to provide video evidence of the problem to Teledyne along with one of the problem cells. Consequently, Teledyne advised us of their belief that some cells have been shipped with the pressure balancing holes blocked.

The electronics board inside the oxygen cells is conformal coated on the R22-2BUD (and other Teledyne variants). The purpose of the conformal coat is to provide a level of waterproofing to the components on the board, should the lid be accidentally flooded.

A proportion of the cells shipped have been over-conformal coated resulting in a thick layer of resin; blocking the two holes in the board.

These are pressure-balancing holes and are important for the "reactivity"; of the cell. If the cell has unequal pressure applied across the front and back end of the cell, then the cell tends to over-react to ambient pressure changes. This results, on the Inspiration, in"Cell Warnings/Errors" being indicated, usually momentarily but occasionally the warnings can persist.

The testing of oxygen cell outputs in air and oxygen tend not to reveal this problem as both tests are done at the same ambient pressure. It is only when you change the ambient pressure will the problem become apparent.

If you have oxygen cells with both holes blocked then the cell should be returned to either the APD factory or Vandagraph, for hole clearing and cell checking. In our inspections we have found cells with only one of the holes blocked but these proved to work just fine. It is requested that one day be allowed for doing the job and a minimum further 2 days for posting back (UK). So in all, you need to allow 5 days even if 1st class post is used.

Ambient Pressure Diving

i) The recommendation is to check the oxygen cells in your rig, particularly if you are getting nuisance cell warnings/errors, to ensure there is at least one pressure balancing hole in the circuit board inside the oxygen cell.

                           
ii) Further to the tests and inspections done at the APD factory it has come to our attention that some customers are sealing the red caps onto the back end of the oxygen cells and onto the black sensor cable by using liquid silicon rubber/sealant, silicon grease or tape, in the belief that moisture is causing their cell warnings. What this actually does is slow down the pressure balancing and exacerbates the cell warnings/errors.

iii) In extreme pressure drops, we have found that occasionally the pressure balancing cannot take place quick enough with the production red cap/ cell arrangement, which results in an occasional nuisance cell warning/error. This is something we have experienced on our test rig, as opposed to diving with it when the cells are subjected to much more gradual pressure changes. In order to ensure fast pressure equalisation across the cell, we have taken the step of putting a 2mm hole in the red cap of each cell.

This is something that you can do with a 2mm diameter drill bit, twist it through the soft material gently using your fingers ( do not use a power tool). It is important to only have one 2mm hole in each red cap as this prevents water flooding into the cell in the event of a serious lid flood. If you decide to undertake this modification yourself please be sure to not damage the wires to the cell. If in doubt whether to drill the 2mm hole, try a dive with the red caps removed (slide them back up the wires) and see if the cell warning problem is eliminated or not.    


Martin

Martin Parker
Managing Director
Ambient Pressure Diving
Manufacturers of the Inspiration Closed Circuit Rebreather
Water-ma-Trout Industrial Estate, Helston, Cornwall, TR13 OLW, UK.
Tel: 01326 563834, Fax: 01326 573605.
Website: www.ambientpressurediving.com